1. Field of the Invention
The present invention relates to the field of power station technology. It refers to a combination power station with power/heat cogeneration, having at least one gas turbine driven by the combustion gases from a combustion chamber and at least one steam turbine working in a water/steam circuit, within the water/steam circuit the water being converted into steam in a first apparatus by means of the hot combustion gases emerging from the at least one gas turbine and the generated steam being supplied to the at least one steam turbine, and a separate heat transfer medium, which is in exchange with a heat consumer, being charged in a second apparatus with thermal energy which is extracted from the water/steam circuit at a plurality of extraction points located at a different thermal level.
Such a combination power station is known, for example, from DE-C2-25 12 774.
2. Discussion of Background
In the case of combination power stations with power/heat cogeneration, the thermal energy always has to be provided in the form required at the respective point in time, that is to say as thermal output for current generation in the generator and as heat-charged heat transfer medium for the external heat consumer or heat consumers. Both the energy demand for current generation and the energy demand for the external heat consumer or heat consumers therefore fluctuate between various extreme values. In particular, according to FIG. 1 in the graph of the temperature (of the heat transfer medium) against the thermal output Pth, the thermal energy required by the external heat consumer fluctuates within a working range (represented by hatching and delimited by the corner points a-d) between a minimum inlet temperature VTmin and a maximum inlet temperature VTmax and between a minimum thermal output Pthmin and a maximum output Pthmax.
The thermal energy (heat) required for the heat consumer is transmitted from the water/steam circuit to the heat transfer medium (water or thermal oil) or to the heat consumer by means of direct or indirect heat transmission processes (by direct contact, for example by mixing, or indirectly, for example via heat exchangers). In order to obtain a working range lying between two extreme values, in this case the thermal energy is extracted from the circuit at a plurality of different thermal levels and is correspondingly transmitted to the heat transfer medium at different thermal levels by means of associated heat exhangers, usually condensers. A maximum number of extraction levels or extraction points results in a minimum loss of energy and maximum thermal efficiency. In the combination power station known from the publication initially mentioned, steam is extracted from the water/steam circuit at various points upstream and downstream of the steam turbine and is supplied in a plurality of individual heat exchangers (heating water heaters) which are connected in series with respect to the flow of the heat transfer medium and heat the heat transfer medium in succession. In this case, the last heat exchanger in the direction of flow is charged directly with fresh steam via a reducing station.
A disadvantage, here, is that, with an increasing number of extraction points and extraction levels, the number of corresponding plant components, such as heat exchangers, lines, control devices and the like, also increases and entails high capital costs as well as operating and maintenance costs. Furthermore, the extreme values of the power demand of the heat consumer, that is to say, for example, the maximum thermal output Pthmax at the maximum inlet temperature VTmax (point c in FIG. 1), do not occur very often or are usually only of short duration, so that the plant components necessary for this purpose are utilized to only a very low degree.